Thermal dye sublimation transfer also called thermal dye diffusion transfer is a recording method in which a dye-donor element provided with a dye layer containing sublimating dyes having heat transferability is brought into contact with a receiver sheet and selectively, in accordance with a pattern information signal, heated with a thermal printing head provided with a plurality of juxtaposed heat-generating resistors, whereby dye from the selectively heated regions of the dye-donor element is transferred to the receiver sheet and forms a pattern thereon, the shape and density of which is in accordance with the pattern and intensity of heat applied to the dye-donor element.
A dye-donor element for use according to thermal dye sublimation transfer usually comprises a very thin support e.g. a polyester support, one side of which is covered with a dye layer, which contains the printing dyes. Usually an adhesive or subbing layer is provided between the support and the dye layer. Normally the opposite side is covered with a slipping layer that provides a lubricated surface against which the thermal printing head can pass without suffering abrasion. An adhesive layer may be provided between the support and the slipping layer.
A dye-image receiving element for use according to thermal dye sublimation transfer usually comprises a support, e.g. paper or a transparent film, coated with a dye-image receiving layer, into which the dye can diffuse more readily. An adhesive layer may be provided between the support and the receiving layer. A release agent may be contained in the receiving layer or in a separate layer on top of said receiving layer to improve the releasability of the receiving element from the donor element after transfer is effected.
The dye layer can be a monochrome dye layer or it may comprise sequential repeating areas of different colored dyes like e.g. of cyan, magenta and yellow hue. When a dye-donor element containing three or more primary colored dye areas is used, a multicolor image can be obtained by sequentially performing the dye transfer process steps for each color area.
Black colored images are obtained by thermal dye sublimation transfer printing either by sequentially performing the dye transfer process steps for the three primary colors cyan, magenta and yellow using a dye-donor element comprising sequential repeating areas of cyan, magenta and yellow colored dyes or by performing only one transfer step using a dye-donor element having a black colored dye layer containing a mixture of yellow, magenta and cyan colored image dyes. The latter of these two methods is preferred due to i.a. ease of manufacturing the donor element containing only one dye area, less time consuming recording with only one transfer step and avoiding the problem of transfer in register of the respective dyes in the respective dye areas. Mixtures of yellow, magenta and cyan dyes for the formation of a black colored dye layer of such a black colored dye-donor element are described in e.g. European patent application no. 92202157.1, EP 453020, U.S. Pat. No. 4,816,435 and JP 01/136787.
One important application of recording monochrome black images by thermal dye sublimation transfer is the recording on transparent film receiver of hard copies of medical diagnostic images obtained by e.g. ultrasound techniques. Such a hard copy is seen as a more ecologically acceptable and more convenient substitute for the black-and-white silver hard copy obtained by developing conventional photographic silver halide film materials where a lot of silver salt containing processing solution has to be treated carefully before disposal.
To be a real valid substitute for conventional photographic silver halide materials the black colored mixture of organic dyestuffs used in thermal dye sublimation transfer printing should behave visually as black silver.
In the medical world, physicians and radiologists use to evaluate their rontgenograms or other images on a light box or negatoscope. These light boxes contain fluorescent lamps as light source. Contrary to incandescent lamps such as tungsten, where the spectral emission is continuous over the whole range of the visual spectrum, the emission of fluorescent lamps depends on the phosphors used in the fluorescent lamp, said phosphors having peak emissions. As a result fluorescent lamps do not show a continuous emission spectrum. Furthermore, there is no standardization in the type of fluorescent lamp used in said negatoscopes.
There is not so much a problem when viewing classical medical images composed of silver metal on the light boxes, the spectral absorption characteristics of silver being constant over the whole visible spectrum. The hue of the silver image does not change whatever the spectral properties of the light source with which the image is viewed.
But when the black image is composed of colored dyes, the spectral absorption characteristics of organic dyestuffs not being constant over the whole range of the visible spectrum, problems of hue changes arise. A black colored dye mixture looking neutrally when viewed with one light source will no longer do so when viewed with a spectrally different light source. This phenomenon of hue change of an image when viewed with a different light source is highly unwanted, especially when evaluating medical diagnostic images.
This hue change can also depend on the density of the image, for example, a rather small hue change in the low density areas and a greater hue change in the high density areas.
The phenomenon of two samples (e.g. metallic silver and a black colored mixture of dyes) that match each other in color but are different in spectral composition is also referred to in literature as metamerism (see, for example, "Measuring Colour" by R. W. G. Hunt, edited by Ellis Horwood Limited, 1987, Chapter 7, Section 7.3, page 141). The greater the degree of metamerism the greater will be the likelihood that the colours will no longer match one another when the illumination is changed.
It is an object of the present invention to provide black colored dye mixtures with an acceptable degree of metamerism relative to metallic silver.
It is another object of the present invention to provide black colored dye mixtures of which the degree of metamerism relative to metallic silver remains substantially the same over the whole density range.
It is another object of the present invention to provide black colored dye mixtures for recording hard copies of medical diagnostic images by thermal dye sublimation transfer printing that behave visually substantially as metallic silver.
Other objects will become apparent from the description given hereinafter.